Translation:I hurt your arm and I want to put on a bandage.
"Seu" is either "your" or "his". I was marked wrong for using "his" instead of "your". Why is this ?
It is correct, but "dele" or "dela" would be much more natural to express "his" or "her". Perhaps this is the reason.
The good thing about such sentences where the word to learn is out of context, is that I'll remember the meaning of it forever (probably both meanings). I will remember that curativo means dressing as well as bandage. Whereas a sample sentence where it can easily be understood from the context that curativo is obviously dresssing doesn't help much, because if I hear the word curativo in a similar, but a bit complexer context next time (which is what the real life offers most of the time) I will recognize the context (thinking: hey I heard curativo in this context before) and won't be able to recall the meaning of curativo. I am supportive of Duolingo's approach, as long as it is not too often repeated, causing alienation.
I like my answer: "I hurt your arm and I want to make a bandage for it." It sounds more natural, more likely to be spoken.
Is CURATIVO really a bandage or plaster? Why not just I WANT TO MAKE IT BETTER?
Yes, "curativo" is a bandage or a plaster. What you suggested would not be suitable for this Portuguese sentence.
Why is my English answer marked wrong: "I hurt your arm and i want to put a bandage on"
I am a German native speaker, but In so many other cases it was perfectly valid to move the "on" behind the object.
Is this 100% incorrect and does not sound any valid?
You need the "it" because if you put something on (with no object for the preposition), we understand it to mean "I'm going to put it on myself."